The title askes the question "Is it Legal?" As far as I am aware it is legal. OK, if a vehicle is modified in any way, then the owner is duty bound to notify the insurance company and may have to pay increased premiums which I doubt. As far as the MOT is concerned, I have not heard of a vehicle failing the emissions test because the DPF has been removed. If it comes into law that DPF removal is illegal, then the simple answer is to remove the guts of the thing and replace the empty shell back onto the vehicle. I share the general view that the DPF system is not fit for purpose.